1. Field of the Invention
Embodiments of the present invention are generally directed toward the use of diamondoids as structural components in nanotechnology, and the combination of one or more structural components to form molecular subsystems. In particular, the present invention is directed toward exemplary diamondoids as rods, screws, brackets, and gears, and the combination of one or more components to make subsystems that include atomic force microscope tips, molecular tachometers and signal waveform generators, and self-assembling cellular membrane pores and channels.
2. State of the Art
Carbon-containing materials offer a variety of potential uses in nanoscale construction. A review of carbon's structure-property relationships has been presented by S. Prawer in a chapter titled “The Wonderful World of Carbon,” in Physics of Novel Materials (World Scientific, Singapore, 1999), pp. 205-234. Prawer suggests the two most important parameters that may be used to predict the properties of a carbon-containing material are, first, the ratio of sp2 to sp3 bonding in a material, and second, microstructure, including the crystallite size of the material, i.e. the size of its individual grains. Elemental carbon has the electronic structure 1s22s22p2, where the outer shell 2s and 2p electrons have the ability to hybridize according to two different schemes. The so-called sp3 hybridization comprises four identical σ bonds arranged in a tetrahedral manner. The so-called sp2-hybridization comprises three trigonal (as well as planar) σ bonds with an unhybridized p electron occupying a π orbital in a bond oriented perpendicular to the plane of the σ bonds.
At the “extremes” of crystalline morphology are diamond and graphite. In diamond, the carbon atoms are tetrahedrally bonded with sp3-hybridization. Graphite comprises planar “sheets” of sp2-hybridized atoms, where the sheets interact weakly through perpendicularly oriented π bonds. Carbon exists in other morphologies as well, including amorphous forms called “diamond-like carbon,” and the highly symmetrical spherical and rod-shaped structures called “fullerenes” and “nanotubes,” respectively.
Diamond is an exceptional material because it scores highest (or lowest, depending on one's point of view) in a number of different categories of properties. Not only is it the hardest material known, but it has the highest thermal conductivity of any material at room temperature. It displays superb optical transparency from the infrared through the ultraviolet, has the highest refractive index of any clear material, and is an excellent electrical insulator because of its very wide bandgap. It also displays high electrical breakdown strength, and very high electron and hole mobilities.
Diamond is an attractive material for other reasons. At room temperature, the root-mean-square amplitude of vibration for diamond is as low as 0.002 nanometers, whereas for other materials this parameter is significantly higher. For example, the element lead (Pb) exhibits a root-mean-square amplitude of vibration of 0.028 nanometers. Low vibrational amplitudes are an important property for precision construction and operation of NEMS (nanoelectromechanical systems).
The microstructure of a diamond and/or diamond-like material further determines its properties, to the extent that microstructure influences the type of bonding content. As discussed in “Microstructure and grain boundaries of ultrananocrystalline diamond films” by D. M. Gruen, in Properties, Growth and Applications of Diamond, edited by M. H. Nazaré and A. J. Neves (Inspec, London, 2001), pp. 307-312, recently efforts have been made to synthesize diamond having crystallite sizes in the “nano” range rather than the “micro” range, with the result that grain boundary chemistries may differ dramatically from those observed in the bulk. Nanocrystalline diamond films have grain sizes in the three to five nanometer range, and it has been reported that nearly 10 percent of the carbon atoms in a nanocrystalline diamond film reside in grain boundaries.
A form of carbon not discussed extensively in the literature are “diamondoids.” Diamondoids are bridged-ring cycloalkanes that comprise adamantane, diamantane, triamantane, and the tetramers, pentamers, hexamers, heptamers, octamers, nonamers, decamers, etc., of adamantane (tricyclo[3.3.1.13,7]decane), adamantane having the stoichiometric formula C10H16, in which various adamantane units are face-fused to form larger structures. These adamantane units are essentially subunits of diamondoids. The compounds have a “diamondoid” topology in that their carbon atom arrangements are superimposable on a fragment of an FCC (face centered cubic) diamond lattice.
Diamondoids are highly unusual forms of carbon because while they are hydrocarbons, with molecular sizes ranging in general from about 0.2 to 20 nm (averaged in various directions), they simultaneously display the electronic properties of an ultrananocrystalline diamond. As hydrocarbons they can self-assemble into a van der Waals solid, possibly in a repeating array with each diamondoid assembling in a specific orientation. The solid results from cohesive dispersive forces between adjacent C—Hx groups, the forces more commonly seen in normal alkanes. That diamondoids have unusual strength and rigidity as individual molecules is made clear from their means of isolation: they survive high-temperature pyrolysis that converts all other hydrocarbons materials in a petroleum feedstock to methane and graphitic carbon.
Buckminsterfullerene (C60), and related nanometer-sized carbon structures (nanotubes) have been applied in the construction of NEMS. Diamondoids have sizes comparable to that of buckminsterfullerene and properties that are complementary. Nanotribology studies have shown that the coefficient of friction of diamond can be over an order of magnitude less than that of buckminsterfullerene, making diamond a preferable material for extended operation of nanometer-sized devices where the contact between two surfaces is a key feature.
Diamond has been shown to be a highly desirable material for the construction of Micro-electrical mechanical (MEMS) micrometer-sized devices. Constructing MEMS out of diamond extends expected operational lifetimes by a factor of 10,000 over MEMs constructed of other materials, e.g., polysilicon. Diamond can similarly be expected to be a highly desirable material for construction of Nano-electrical mechanical systems (NEMS), nanometer-sized devices.
The use of “diamondoids” as structural components in nanoscale technologies has been discussed by K. E. Drexler in Nanosystems (Wiley, New York, 1992), pp. 253-272. Drexler states that the strength, stiffness, shape, and surface properties of nanoscale components determine what they can do. In this chapter, Drexler discusses components from a structural perspective, noting that while it is natural to focus on moving parts, much of a typical system has its mass in the form of a stiff housing. Gears, bearings, springs, screws, sliding rods, and motors should be pictured as being anchored to or embedded in an extended diamondoid structure, the structure tailored to support the components in functional positions with respect to one another.
Drexler was first to identify the qualities of diamondoid structures for construction of nanometer-sized devices. He envisioned complex nanometer-sized diamondoid objects, such as bearings, that are analogs of macroscopic machine parts, and calculated their properties using advanced computerized molecular simulations. This work has been extended by Merkle and others, but is still largely a theoretical projection due to a lack of actual materials and methods. The difficulty is that the diamondoid structures that were imagined would be virtually impossible to prepare using current technologies. Only one of the four possible diamondoids having four face-fused adamantane subunits (i.e., only one of the tetramantane isomers) has been synthesized to date, and then only with great difficulty.
Along similar lines, the concept of molecular manufacturing and molecular engineering has been reviewed by Z. Asfari and J. Vicens in a review article titled “Molecular Machines,” J. of Inclusion Phenomena and Macrocyclic Chemistry, Vol. 36, pp. 103-118 (2000). Molecular manufacturing and molecular engineering are approaches to the development of general capabilities for molecular manipulation to produce new organic and biological materials manufactured atom by atom at the molecular level. The terms “molecular-size tinkertoy construction” and “molecular lego” have been used in the literature to characterize this branch of organic chemistry. And their review chapter, Asfari and Vicens described molecular-size systems exhibiting mechanical properties that may be interpreted in terms of classical mechanics. In other words, a molecular size system may be thought of as an assemblage of parts designed to transmit or modify the application of power, force, or motion to other parts of the system in a predetermined manner. The mechanical properties of these components are related to their geometries, their ability to thermally rotate around single bonds, steric effects amongst components of the system, and the manner in which forces are translated through the rigid architectures provided by the system. Examples of molecular-size systems reviewed by Asfari and Vicens include propellers, gears, beveled gears, toothed cogs, brakes, ratchets, turnstiles, pendulums, gyroscopes, rotors, impellers, and shuttles.
The importance of rotary motion in such nanoscale systems is brought to light in another review article titled “Rotary motion in single-molecular machines” by T. R. Kelly and J. P. Sestelo, in Molecular Machines and Motors, J.-P. Sauvage, ed. (Springer, Berlin, 2001), pp. 19-51. The chapter focuses on molecular systems that exhibit controlled or coordinated rotary motion, and emphasizes how such systems represent a reproduction of a variety of macroscopic mechanical devices on a molecular scale. Examples of such systems described by the authors include molecular gears, turnstiles, brakes, ratchets, and rotary motors.
Drexler's definition of the term “diamondoid” is broad in comparison to its use in the present patent. Nanosystems defines the term as a “strong, stiff, covalent solid with a dense, three-dimensional network of bonds.” Drexler states that the diamondoid solids of most interest have compositions that include multivalent elements from the first row of the periodic table, such as boron, carbon, nitrogen, and oxygen, but may make substantial use of similar second row elements such as silicon, phosphorus, and sulfur, and limited use of monovalent covalent elements such as hydrogen, fluorine, and chlorine. Included in the definition of diamondoid materials are silicon carbide, alumina, silicon nitride, and tungsten, with the properties shown in Table I:
TABLE IProperties of diamondoid materials reported by DrexlerYoung's ModulusStrengthDensityMaterial(GPa)(GPa)(kg/m3)Diamond1050503,500SiC700213,200Al2O3532154,000Si3N4385143,100Tungsten350419,300
Drexler also points out that such diamondoid structures have a further advantage in that they are amenable to a description by molecular mechanical modelling, including the property stiffness.
Finally, Drexler states that it would be useful to “specify and characterize many small, regular structures useful as shafts, gears, and so forth; means for indexing and recovering designs are of comparable importance.” What is needed is a selection of diamondoid materials that have been cataloged in terms of shape and dimensions.